The present invention relates to devices suitable for use on board an aircraft or a spacecraft and which are intended to measure the backscattering of the light by a remote scene.
It relates more particularly to the devices of this type, called lidars, comprising a laser for emitting short light pulses, a channel for receiving the backscattered light flux and optical conical scanning means making it possible to orient a direction common to laser emission and to reception via the said channel along successive generatrices of a cone.
An important application of the invention is in the field of lidars on board a satellite and intended for meteorological exploration of the Earth. These lidars emit a laser pulse, of from a few nanoseconds to a few microseconds in length, through a telescope. The wave backscattered by the aerosol particles in the atmosphere makes it possible to measure parameters such as the aerosol concentration or the wind velocity. Since the successive layers in the atmosphere are reached by the laser pulse at different times, splitting the return signal in time slots makes it possible to map the parameters through the thickness of atmosphere probed.
The lidars used may be incoherent: they then just supply information about the variation in the aerosol density as a function of the altitude and about the altitude of the cloud tops, unless the Doppler shift is detected by a spectrometer such as a Fabry-Perot spectrometer.
More often, coherent lidars or Doppler lidars are used which allow direct measurement of the wind velocity by the Doppler effect and by heterodyne detection, the backscattered wave undergoing a frequency shift proportional to the radial velocity relative to the target. The wind velocity at a given altitude is then obtained by subtracting, from the measured relative radial velocity, the contributions of the speed of the craft carrying the device and of the velocity due to the Earth's rotation.
From a Doppler measurement, only the projection of the velocity vector on the viewing direction of the instrument is obtained. A device of the above kind, for meteorological applications, is consequently advantageously designed to measure at least two components of the velocity, separated by approximately 30.degree..
Devices of the above kind, mounted on a low-orbit satellite, have already been proposed.
The line of sight of an emitter telescope stationary with respect to the satellite describes a curve over the Earth's surface which reproduces the track of the satellite. In order to obtain good coverage of the Earth, it is necessary that the device probe the atmosphere on either side of the track of the satellite on the ground. To do this, it has been proposed either to move the telescope assembly or to add to the telescope a scanning device located in front of it, such as a flat mirror, providing conical scanning, which corresponds to a cycloid-shaped scanning trace on the ground.
These two known types of scanning have serious drawbacks from the point of view of the mechanical stresses imposed on the satellite's platform. If the telescope is moved in its entirety, it generates reaction forces on the platform, hence stresses and the necessity for compensation in order to maintain attitude control of the satellite. If a mirror or prism is moved in front of the satellite, it must be very large in order to allow a significant aperture.
In practice, this prevents the use of a stepper drive: the telescope or the mirror rotates continuously. This causes two parasitic effects. The backscattered wave is not recovered in the same direction as the transmitted wave, thereby creating an angular aberration phenomenon; since the pulse takes a finite time to pass through the atmosphere, the direction of the return wave varies throughout the duration of the return signal, thereby resulting in blurring.
A telescope with conical scanning has also been proposed [U.S. Pat. No. 5,071,239 to Hoffman et al], comprising an objective, a mirror having a configuration of an ellipsoid ring and a continuously rotating mirror located on the axis of the cone.